In effect, piloting any aircraft entails knowing its relative speed in relation to the air, that is to say to the relative wind. This speed is determined using probes for measuring the static pressure Ps and the total pressure Pt. The total Pt and static Ps pressures provide the modulus of this speed vector.
As is known, the total pressure Pt can be measured using a so-called Pitot tube. This is a tube that is open at one of its ends and blocked at the other. The open end of the tube faces substantially into the flow. The stream of air situated upstream of the tube is progressively slowed down until it reaches an almost zero speed at the inlet of the tube. The slowing down of the speed of this stream of air increases its pressure. This increased pressure forms the total pressure Pt of the flow of air. The principle of such a total pressure measurement probe is recalled by FIG. 1. The probe 10 is intended to be fixed through an opening 11 produced in the skin 12 of an aircraft. The probe 10 comprises a part 13 external to the skin 12 and formed by a Pitot tube 14 borne by a strut 15. The probe 10 also comprises an internal part 16 essentially comprising an electrical connector 17 and a pneumatic connector 18. The connector 17 makes it possible to electrically connect the probe 10 to the aircraft, for example to connect heating means for the de-icing of the probe 10. The connector 18 allows for the pneumatic connection of the Pitot tube 14 to a pressure sensor or other measurement device, situated inside the skin 12 of the aircraft. The probe 10 is positioned on the skin 12 of the aircraft such that the Pitot tube 14 is oriented substantially along a longitudinal axis of the aircraft, excluding the laminar boundary layer, for the direction of flow, embodied by an arrow 19, to substantially face an inlet orifice 20 situated at a first end 21 of the Pitot tube 14. In the example represented, the Pitot tube 14 is fixed relative to the skin 12 of the aircraft. It is of course possible to mount the Pitot tube 14 on a mobile strut such as, for example, a paddle that can be oriented in the axis of the flow as for example described in the patent published under the number FR 2 665 539.
In practice, the flow of air can convey solid or liquid particles, such as, for example, water from the clouds, likely to penetrate into the Pitot tube and to build up in the tube at the blocked end. To prevent such a build-up from disturbing the pressure measurement, one or more drain holes and also water traps are generally provided, to avoid any risk of blocking of the ducts responsible for transmitting the total pressure to the pressure sensors situated inside the skin of the aircraft or to the instruments of the aircraft instrument panel. As represented in FIGS. 2a and 2b, the Pitot tube 14 thus comprises, in proximity to an end 22, one or more drain holes 23a and 23b that make it possible to discharge particles likely to penetrate into the tube 14. Still at the end 22 of the tube, an air line 24 opens into the tube 14 to there form a pressure tap 40 at which the air pressure is to be measured. The pressure tap 40 is generally constructed in such a way as to avoid the ingestion of water into the tube 14 and thus form a water trap. The line 24 is, for example, linked to a pressure sensor that is not represented in FIG. 2. The pressure sensor makes it possible to effectively measure the pressure of the air prevailing inside the tube 14 at its end 22. Apart from the drain hole or holes, whose sections are small compared to that of the tube 14, the tube 14 is closed at its end 22. The pressure measured at this end therefore represents the total pressure Pt of the flow of air.
The drain holes make it possible to discharge the liquids and any particles that might penetrate into the tube. The slowing down of the air in the tube is not therefore complete and the total pressure Pt measurement is thereby affected. More specifically, the greater the effort to avoid the build-up of water or of particles of significant size, the more the total pressure measurement is thereby affected by increasing the dimensions or the number of drain holes. Conversely, the greater the effort to enhance the total pressure Pt measurement by reducing the dimensions or the number of drain holes, the greater the risk of build-up of water or of particles. With a Pitot tube, there therefore has to be a trade-off between quality of the total pressure Pt measurement and risk of disturbance of the measurement because of the penetration of water, and of particles conveyed by the flow of air where the measurement is performed.
In the operational life of aircraft, the drain holes can be polluted, because of the ingestion of dust, insects, plant residues or other foreign bodies. Because of their size and the position of the Pitot tubes on the fuselage of an aircraft, the periodic checking of the integrity of the drain holes is difficult.
The checking of the drain holes of the Pitot tubes is generally done visually. The operator responsible for the maintenance of the aeroplanes inspects the drain hole or holes using a small lamp. If foreign bodies are observed, the probe is dismantled, and its pneumatic circuits cleaned. This operation is all the more difficult when the aeroplane is of large size. Access to the probe and to the drain holes whose diameter is generally less than 1 mm is difficult.
Also known from the applicant is a checking device intended to be connected temporarily to the pressure measurement probe, and that makes it possible to check, using an acoustic transmitter and an acoustic receiver, the non-blocking of the internal cavities and of the drain holes of the probe. The principle of such a device is notably described by the patent published under the reference FR 2 959 822. FIGS. 2a and 2b of this application also recall this principle. The checking device 25 comprises a transmitter 26 and a receiver 27 intended to be connected to an internal volume 30 of the probe, formed by the inside of the tube 14, the drain hole or holes 23a and 23b, and the line 24. The transmitter transmits an acoustic signal that is propagated in the internal volume 30 and the receiver is configured to pick up an acoustic signal observed in the internal volume 30. The device also comprises processing means 28 that make it possible to compare the acoustic signal observed in the internal volume to a reference acoustic signal measured on a non-clogged probe, in order to establish the presence of particles in the internal volume.
The comparison of the acoustic signal measured during a periodic maintenance operation to a predefined reference acoustic signal in practice comes up against a number of difficulties. The acoustic signal picked up by the receiver is greatly dependent on the conditions of the measurement. For example, the ambient temperature which affects the propagation of the acoustic signal directly impacts on the effectiveness of the comparison. Dispersions, even when they are limited, in the geometry of the internal volume also impact on the measured acoustic signal. The direct comparison of an acoustic measurement performed during a maintenance operation for a probe mounted on an aircraft, with an acoustic signal measured in a laboratory on a reference probe does not make it possible to effectively detect the blocking of the drain holes of a pressure measurement probe.